The invention relates to a method for operating a parallel hybrid drive for a motor vehicle having an internal combustion engine connected to a drive shaft via a clutch and a manual transmission, and a three-phase machine with a rotor which is directly coupled to a countershaft of the manual transmission, and which is connected to an electrical energy storage unit via a three-phase converter.
A hybrid drive of this type for a motor vehicle is disclosed in German patent document DE 42 02 083 A1. In addition to the three conventional operating modes of the hybrid drive (one or the other of the two drive units used alone, or both used together), it is proposed to employ the three-phase machine as a synchronization aid for the manual transmission during gear changing.
Preliminary publication DT 23 53 724 discloses a parallel hybrid drive in which a heat engine is operated in a fixed rotational speed relationship with a gyro energy storage unit and, together with an electrical machine, can be coupled to a power-split gear. The power capacity of the electrical machine is dimensioned appreciably lower than that of the gyro storage unit and the heat engine. The heat engine is to be operated in favourable characteristic map ranges and is to be regulated with delayed action (that is, slowly). Brief deviations in the power required from the hybrid drive are covered by the gyro storage unit and the electrical machine.
German patent document DE 44 22 636 A1 discloses a method for automatically controlling a parallel hybrid drive, in which the power of an internal combustion engine is supplied in three different ways as a function of current driving parameters; in particular the power requirement, the vehicle speed and the battery charge state. Various special values to be determined serve as evaluation criteria, including power limits of the drives and of the battery and also speed limit values. The internal combustion engine is operated as a function of these criteria. At the same time, in principle, the supply of power by the internal combustion engine is to be minimized for reasons of exhaust-gas emissions, without the elasticity of the overall unit being restricted so as to impair driving comfort.
The publication by B. Giera, et al., "Hybridantrieb mit Gryo-Komponente fur wirtschaftliche und dynamische Betriebsweise" ["Hybrid drive with gyro component for efficient and dynamic operation"], ETZ-A No. 94 (1973), page 653, describes a hybrid drive with an internal combustion engine, a gyro component and an electric motor. The electric motor is connected to a first input of a variable-ratio differential gear, while a second input of the latter has coupled to it the internal combustion engine and the gyro component connected in parallel to the latter by means of a step-up. The output of this differential gear is connected via a manual transmission to a conventional differential gear of the driving axle. For emissions reasons, the power output of the internal combustion engine is regulated only very slowly, in a desensitized manner. The maximum specific power of the gyro component is selected to approximate a multiple of that of the internal combustion engine, which, in turn, is typically about twice as high as that of the electric motor.
German patent document DE 44 22 647 A1 discloses a method for automatic control of the supply of power by an internal combustion engine and/or by an electric motor capable of being driven via an electrical energy store, in motor vehicles with a hybrid drive. When the required drive power is no higher than a predetermined basic power, it is supplied solely by the internal combustion engine; howeyer, in operating phases in which the required drive power is higher than the predetermined basic power, if the electrical energy store is sufficiently charged, some of the drive power is supplied additionally by the electric motor. Preferably, the predetermined basic power is the maximum power output of the internal combustion engine as a function of the rotational speed prevailing in each case. When the required drive power is lower than the predetermined basic power and the electrical energy storage unit is not sufficiently charged, excess power is supplied by the internal combustion engine in order to charge the electrical energy store.
The journal article by P. Chudi and A. Malmquist, Schadstoffarmes Hybridtriebwerk fur moderne Lastkraftwagen und Omnibusse [Low-pollutant hybrid engine for modern heavy goods vehicles and buses], ABB Technik 6/7, 1996, page 12, discloses a serial hybrid drive of special design, as well as a suitable operating mode. A gas turbine is provided as the internal combustion engine. In order to control the hybrid drive, a vehicle management computer detects and integrates the instantaneous power requirement to determine an average power requirement for the electric drive motor. The desired value for the gas turbine power is then set equal to the sum of this average power requirement of the electric drive motor and a power component for recharging batteries functioning as electrical energy storage units. A high-speed generator is coupled mechanically to the gas turbine. The electric drive motor is then connected electrically, on the one hand, to the high-speed generator and, on the other hand, to the batteries chargeable by the latter, so that the motor is fed in parallel by the high-speed generator, together with the coupled gas turbine, and by the batteries.
One object of the present invention is to provide a method for operating a parallel hybrid drive of the type described above which ensures by the simplest possible means that the internal combustion engine is operated with emissions which are as low as possible.
This object is achieved by the method according to the invention, in which a time average of the driving torque required during a respective predeterminable travel time interval is determined by control, and is demanded as load from the internal combustion engine. The difference between the driving torque thus delivered by the internal combustion engine and the driving torque currently required is made available or absorbed by the three-phase machine, which is connected via a three-phase converter to an electrical energy storage unit. Drive energy required for a short time can thus be extracted from the storage unit and stored intermediately in the braking energy. With continuous sliding averaging of the required driving torque, load jumps are transmitted to the internal combustion engine without delay, but in a leveled manner; if appropriate, the three-phase motor supplies the resulting brief power demand peaks. As a result, the internal combustion engine is operated in a desensitized manner, and the extent of desensitization can be set by a suitable choice of the length of the travel time interval used for averaging.
In one embodiment of the method according to the invention, the travel time interval for averaging is determined as a function of the emission behaviour of the internal combustion engine under load changes and/or of the current operating state of the latter. It is selected sufficiently large that, in the case of a given maximum load jump, the load change of the internal combustion engine (and therefore the pollutant emissions) do not exceed a specific tolerance value. The travel time interval may be fixed in this manner, separately for a plurality of operating states of the internal combustion engine, since the actual absolute load on the internal combustion engine is of greatest importance. Since the timespan of typical acceleration operations of a vehicle in town traffic (particularly a commercial vehicle) is generally on the order of magnitude of 15 s, the travel time interval for averaging is selected preferably on this order of magnitude.
According to another embodiment of the invention, during constant travel with the internal combustion engine is operated below full load, the electrical storage unit is charged because the internal combustion engine delivers power greater than necessary for the driving mode. Energy conversion losses can thereby be compensated so as to save fuel.